Straw is particularly useful as animal bedding, horticultural mulch, and for industrial and commercial purposes such as in insulation and packing materials. Straw is produced by cutting standing crop and, normally after stripping off any useful part, spreading it on the field to dry for some time. Hay is the more useful part of the standing crop that is similarly cut and dried in the field, but that is subsequently used as animal fodder or ensilage.
In order to turn the loose straw or hay into a product that can be transported, stored, and accounted for, it is pressed into bales. For commercial purposes the so-called large-format square bale, which is actually parallepipedal, is used. It is produced in a baler that picks the cut crop up off the ground, normally comminutes it somewhat, presses it into bales, applies ties around the bales, and drops the bales on the field behind the machine. In view of the pace of the baling operation, it is normally more expedient for the baler simply to leave a trail of bales in the field that are later picked up by a different crew operating at a different pace.
Ideally, all of the bales produced should be of the same size and weight. The size is determined by the bale length, width, and height. The width and height are established by the size of the pressing passage and the length by the cycling time of the tying device of the baler. The weight is clearly a function of size, and also of the density of the bale, that is how tightly or loosely the straw or hay is packed in it.
The press passage typically has side walls that can be moved horizontally by hydraulic actuators to narrow or widen the width of this passage. Since the downstream end of the passage is always open, it is the friction with the sides of the passage that determines how much pressure resists the piston that packs wads of the straw or hay against the trailing end of the strand that is subdivided by the tying device into bales while still in the passage. Thus as the passage narrows, the friction increases as does the density of the bale and in fact it is the relative spacing of these side walls that is a principal factor, along with the consistency of the material being baled, in the density of the bales being produced.
German 38 20 367 describes a large-format square baler that picks the crop off the ground and packs it in a press passage in which a piston is reciprocated by a crank drive driven from a tractor power takeoff. The drive is supported at its upper side on a beam that is fixed on the side walls of the pressing passage. The rear end of the pressing passage is as described above formed by pivotal wall panels whose spacing can be changed hydraulically. The front wall of the pressing passage carries an inductive position sensor spaced closely to the beam so that its output indicates the amount of bend in this beam which is generally proportional to the force being exerted by the piston. This output is amplified and fed to a threshold discriminator. The output of the amplifier is smoothed and fed to an adding circuit and there compared to a set point. The position of the side-wall panels is adjusted in accordance with the difference.
Such an arrangement makes it possible to maintain the pressing force generally constant but does not really provide a readout of the actual density of the bales being produced. This density is affected by many factors, including the piston force, the composition of the material being baled, the moisture content of the material being baled, and the friction of the walls of the press passage.
European 0,223,350 and 0,223,351 describe an improvement on the system of DE 38 20 367. Here the crank drive for the pressing piston has a pair of piston rods that are each mounted at on end on the crank on the output shaft of the main drive and at the other end on bolts on the pressing piston. These bolts are provided with shear-force sensors in an annular array or with bend-type strain gauges. Thus the pressing force is measured directly, but as mentioned above this is only a factor in the bale's density and does not provide the equipment operator with an exact readout of bale density.
Finally, German 27 34 766 describes another large-format square baler wherein a pivotal side wall of the pressing passage is associated with a sensor and is urged into the passage so that its deflection is a measure of the compaction in the passage. This system is used with a set-point system as above to establish a standard compaction, but still does not give the operator of the machine a display or readout of the densities of the bales being produced.